Adaptive modulation is a technique used to maximize the capacity of a communication system without affecting its availability. In its most general form a communication system employing adaptive modulation uses a channel estimate derived at the receiver to control transmission parameters such as modulation order and Forward Error Correction (FEC) overhead, with the objective of maximizing traffic capacity.
An adaptive modulation system is depicted in FIG. 1 and includes a transmitter side 1 and a receiver side 2. The transmitter side 1 comprises a Media Access Controller 3a (MAC) for receiving raw data hits from a source of information and scheduling the bits for transmission, with a data rate which is increased or decreased depending on the selected modulation level M. A FEC encoder 4a encodes the bits from the MAC 3, wherein the encoding operation may include scrambling, interleaving, outer and inner code encoding based at block level, or other conventional encoding methods.
An M-point Quadrature Amplitude Modulation (M-QAM) modulator 5a is then provided for mapping the encoded bitstream from the FEC encoder 4a, according to the modulation mode in use, which is based on the number M of points defining the QAM constellation. Before radio transmission, a TX filter section 6a is used for processing and preparing for radio transmission the symbols from the M-QAM modulator 5a. 
The receiver side 2 of the system includes an RX filter section 6b, for performing a first filtering operation on the received radio signal, and an adaptive equalizer section 7 which restores the signal quality by removing the residual inter-symbol interference, for example, by employing both the Constant Modulus (CM) and the Decision Directed (DD) update methods. A soft-decision M-QAM demodulator 5b is also provided in the receiver side 2 for demapping the received symbols and, downstream of the demodulator, a FEC decoder 4b applies FEC error correction to the demapped bitstream from the demodulator 5b using soft information. Finally, a second MAC 3b repacks the variable rate data stream.
The transmitter side 1 has the capability to switch the modulation level M upon receiving a command from the receiver side 2, based on the detected radio propagation conditions. For instance, if the weather conditions in the radio channel worsen, e.g., from sunny to heavy rainy, the QAM modulation level M may be changed from 256-QAM to 64-QAM, because of the capacity drop of the channel.
Systems are known where different types of information available before both symbol demapping and FEC decoding are used as channel estimator. Particularly, it is known to use the Mean Square Error (MSE) level of the signal measured at the receiver as an estimator of the Signal to Noise Ratio (SNR) or the Signal to Noise and Interference Ratio (SNIR). Normally, the MSE is measured and averaged over a time frame of the received radio communication signal, which may cover from 500 microseconds to 3 milliseconds depending on the specific application. The averaged MSE is then compared with thresholds corresponding to the modulation or physical layer (PHY) modes shift levels. In order to avoid bit errors during the shift, an additional margin on the shift levels is added, the margin being computed taking into account a reference SNIR level at a Bit Error Rate (BER) of 10−6.
WO 00/76109, instead, discloses a method which considers both signal strength and the BER at the receiver, after error corrections, for determining the modulation mode.
In WO 99/12302, BER is disclosed as one of the possible means for indicating the quality level.
A drawback affecting the known systems is that a relatively long time period, such as an entire radio frame, is needed to get a reliable estimator of the SNIR.
Moreover, the behaviour of the BER of the uncoded signal and the FEC coding gain curve shape are not tracked, because the use of receiver information before symbol demapping takes into account only modulation information available at the receiver.
Another drawback is that when BER is considered for adaptive modulation purposes, it is the BER available after errors have been corrected, e.g. the BER measured downstream of a FEC decoder, and this implies a very slow reaction to fast variations in the radio channels.